I built a link shortener with FastAPI and htmx (no JS framework) — the parts that were actually hard

"A URL shortener" sounds like a weekend project. Slug in, long URL out, 302 , done. That's what I thought too. Then real usage showed up: links opened inside Instagram's in-app browser and didn't convert, bot traffic wrecked the analytics, and one link needed to send a US visitor somewhere different from an EU visitor. Suddenly the "trivial" part was 5% of the work. I built the whole thing on FastAPI + Redis + MySQL + htmx , deliberately with no frontend framework . This post is about the parts that turned out to be interesting — the redirect hot path, geo/device routing, and escaping in-app browsers — and why htmx was the right call for a one-person team. Disclosure: I build tapurl.io , a link shortener for marketers. This is a write-up of the engineering behind it, not a pitch — everything below is patterns you can apply to any shortener. The redirect is a hot path, so treat it like one Every other page in the app can be a bit slow. The redirect cannot. It sits in front of someone's click, and it runs on every click, so it has to be a tight, predictable read. The naive version hits your database for every redirect: @app.get ( " /{slug} " ) async def redirect ( slug : str ): link = await db . fetch_link ( slug ) # DB round-trip on every click if not link : raise HTTPException ( 404 ) return RedirectResponse ( link . destination , status_code = 302 ) That's fine until you have traffic. The slug-to-link lookup is a near-perfect cache candidate — a slug maps to the same link record every time. So the real path reads from Redis first and only falls back to MySQL on a miss: async def resolve ( slug : str ) -> Link | None : cached = await redis . get ( f " link: { slug } " ) if cached : return Link . parse_raw ( cached ) link = await db . fetch_link ( slug ) if link : await redis . set ( f " link: { slug } " , link . json (), ex = 3600 ) return link Two things worth saying out loud: Cache the lookup, not the decision. You cache the link record, but the actual destination